Many applications including sensing and membranes rely on efficient and reliable methods for fabricating porous materials thin films. Generally, porous thin films can be fabricated by attaching porous material to a surface, such as a functionalized substrate. It is critical that porous thin film fabrication methods be robust, and capable of generating homogeneous films with well-defined thicknesses. In some cases, a high degree of orientation can be beneficial. Metal-organic frameworks (MOFs) are a new class of crystalline porous materials, which are very well suited for use as surface-modifying coatings. The adaptation of specific functions to MOF thin films can be achieved either by loading deposited MOF with functional molecules or by through a post-functionalizing modification of the MOF-constituents.
Liquid phase epitaxy (LPE) fabrication processes (or layer-by layer methods) can be used to obtain crystalline, highly oriented MOFs layers on substrates. Substrates can include modified Au-substrates, also referred to as SURMOFs. A major drawback of this LPE method, however, is that the sequential deposition process is time consuming and tedious. For example, an LPE method can require 400 separate immersion cycles to fabricate a film 100 layers (i.e., ˜100 nm) thick. Such a process can take approximately 3 days, and further consume large quantities of chemicals and solvents. These drawbacks preclude the use of MOFs from applications which require thicker and/or mechanically strong layers, such as membranes, storage, and small molecule separation (e.g., gas phase chromatography, liquid phase chromatography) where a thickness of at least a 1 μm can be required.
Spray methods can be used to fabricate MOFs at speeds which are two orders of magnitudes faster than LPE fabrication processes. The spray method utilizes a nozzle system which deposits the reactant's solutions and solvents required for the MOF thin film growth onto a targeted surface (i.e., a substrate) in a form of aerosol. The aerosol droplets, having sizes down to about 10 μm, impinge on the substrate and coat the surface with a thin film of the desired reactant. The reactant, either a metal precursor or an organic ligand, deposits at the solid/liquid interface in a similar fashion as with the LPE process. The coated surface is next sprayed with solvent to remove unreacted material. However, nozzle limitations make it difficult to apply a homogenous coating, and the process consumes a large amount of chemicals and solvents, thereby making the process prohibitively inefficient at larger scales.